Linear multivariable systems, which have physical parameters changing their values from the nominal ones in an arbitrary and non-stationary way, are considered. A control plant is subject to action of polyharmonic external disturbances containing an arbitrary number of frequencies with unknown amplitudes (their sum is bounded). The following problem of controller synthesis is formulated: design a controller that guarantees robust stability of the closed loop system and in addition provides given controlled variables errors of the nominal plant for the steady-state process. The problem solution is based on a representation of the system equations in (W;Lambda;K)-form and is formulated as a standard H-inf optimization procedure. Desired accuracy is provided through analytic selection of a weighting matrix for the controlled variables of the plant. The proposed approach is illustrated with a solution of a well-known benchmark problem.